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soundchip.js
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soundchip.js
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function SoundChip(sampleRate, cpuHz) {
"use strict";
var soundchipFreq = 3546893.0 / 16.0; // PAL
var sampleDecrement = soundchipFreq / sampleRate;
var samplesPerCycle = sampleRate / cpuHz;
var register = [0, 0, 0, 0];
this.registers = register; // for debug
var counter = [0, 0, 0, 0];
var outputBit = [false, false, false, false];
var volume = [0, 0, 0, 0];
this.volume = volume; // for debug
var generators = [null, null, null, null];
var volumeTable = [];
var f = 1.0;
var i;
for (i = 0; i < 16; ++i) {
volumeTable[i] = f / generators.length; // Bakes in the per channel volume
f *= Math.pow(10, -0.1);
}
volumeTable[15] = 0;
function toneChannel(channel, out, offset, length) {
var i;
var reg = register[channel], vol = volume[channel];
// For jsbeeb 0 is treated as 1024. However, I found this
// made things like Altered Beast's background music have
// a low note play in the background.
if (reg <= 1) {
for (i = 0; i < length; ++i) {
out[i + offset] += vol;
}
return;
}
for (i = 0; i < length; ++i) {
counter[channel] -= sampleDecrement;
if (counter[channel] < 0) {
counter[channel] += reg;
outputBit[channel] = !outputBit[channel];
}
out[i + offset] += outputBit[channel] ? vol : -vol;
}
}
var lfsr = 0;
function shiftLfsrWhiteNoise() {
var bit = (lfsr & 1) ^ ((lfsr & (1 << 3)) >> 3);
lfsr = (lfsr >> 1) | (bit << 15);
}
function shiftLfsrPeriodicNoise() {
lfsr >>= 1;
if (lfsr === 0) lfsr = 1 << 15;
}
var shiftLfsr = shiftLfsrWhiteNoise;
function noisePoked() {
shiftLfsr = register[3] & 4 ? shiftLfsrWhiteNoise : shiftLfsrPeriodicNoise;
lfsr = 1 << 15;
}
function addFor(channel) {
channel = channel | 0;
switch (register[channel] & 3) {
case 0:
return 0x10;
case 1:
return 0x20;
case 2:
return 0x40;
case 3:
return register[channel - 1];
}
}
function noiseChannel(channel, out, offset, length) {
var add = addFor(channel), vol = volume[channel];
for (var i = 0; i < length; ++i) {
counter[channel] -= sampleDecrement;
if (counter[channel] < 0) {
counter[channel] += add;
outputBit[channel] = !outputBit[channel];
if (outputBit[channel]) shiftLfsr();
}
out[i + offset] += (lfsr & 1) ? vol : -vol;
}
}
var enabled = true;
function generate(out, offset, length) {
offset = offset | 0;
length = length | 0;
var i;
for (i = 0; i < length; ++i) {
out[i + offset] = 0.0;
}
if (!enabled) return;
for (i = 0; i < 4; ++i) {
generators[i](i, out, offset, length);
}
}
var cyclesPending = 0;
function catchUp() {
if (cyclesPending) {
advance(cyclesPending);
}
cyclesPending = 0;
}
this.polltime = function (cycles) {
cyclesPending += cycles;
};
var residual = 0;
var position = 0;
var maxBufferSize = 4096;
var buffer;
if (typeof Float64Array !== "undefined") {
buffer = new Float64Array(maxBufferSize);
} else {
buffer = new Float32Array(maxBufferSize);
}
function render(out, offset, length) {
catchUp();
var fromBuffer = position > length ? length : position;
for (var i = 0; i < fromBuffer; ++i) {
out[offset + i] = buffer[i];
}
offset += fromBuffer;
length -= fromBuffer;
for (i = fromBuffer; i < position; ++i) {
buffer[i - fromBuffer] = buffer[i];
}
position -= fromBuffer;
if (length !== 0) {
generate(out, offset, length);
}
}
function advance(time) {
var num = time * samplesPerCycle + residual;
var rounded = num | 0;
residual = num - rounded;
if (position + rounded >= maxBufferSize) {
rounded = maxBufferSize - position;
}
if (rounded === 0) return;
generate(buffer, position, rounded);
position += rounded;
}
var latchedChannel = 0;
function poke(value) {
catchUp();
var latchData = !!(value & 0x80);
if (latchData)
latchedChannel = (value >> 5) & 3;
if ((value & 0x90) === 0x90) {
// Volume setting
var newVolume = value & 0x0f;
volume[latchedChannel] = volumeTable[newVolume];
} else {
// Data of some sort.
if (latchedChannel === 3) {
// For noise channel we always update the bottom bits of the register.
register[latchedChannel] = value & 0x0f;
noisePoked();
} else if (latchData) {
// Low 4 bits
register[latchedChannel] = (register[latchedChannel] & ~0x0f) | (value & 0x0f);
} else {
// High bits
register[latchedChannel] = (register[latchedChannel] & 0x0f) | ((value & 0x3f) << 4);
}
}
}
for (i = 0; i < 3; ++i) {
generators[i] = toneChannel;
}
generators[3] = noiseChannel;
this.render = render;
this.poke = poke;
this.reset = function () {
for (var i = 0; i < 3; ++i) {
counter[i] = volume[i] = register[i] = 0;
}
noisePoked();
advance(100000);
cyclesPending = 0;
};
this.enable = function (e) {
enabled = e;
};
this.mute = function () {
enabled = false;
};
this.unmute = function () {
enabled = true;
};
}